Carrier WEATHERMASTER 48/50Z030-105 User Manual

Page 59

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Limit Switch Temperature Monitoring (LIM.M) — Variable

air volume applications in the low heat or tempering mode can

experience low airflow and as a result it is possible for nuisance

trips of the gas heat limit switch, thereby shutting off all gas

stages. In order to achieve consistent heating in a tempering

mode, a thermistor (Temperatures

→

AIR.T

→

S.G.LS) is placed

next to the limit switch and monitored for overheating. In order

to control a tempering application where the limit switch

temperature has risen above either the upper or lower configu-

ration parameters (SW.L.T, SW.H.T), the staged gas control

will respond to clamp or drop all gas stages.

If the Limit Switch Monitoring configuration parameter

(LIM.M) is set to YES, all the modes will be monitored. If set

to NO, then only LAT Cutoff mode and Capacity Clamp mode

for RISE will be monitored.

If S.G.LS rises above SW.L.T or if (LAT – LAT last time

through the capacity calculation) is greater than (RISE)

degrees F per second, the control will not allow the capacity

routine to add stages and will turn on the Capacity Clamp

mode.

If S.G.LS rises above SW.H.T the control will run the capac-

ity routine immediately and drop all heat stages and will turn

on the Limiting mode.

If S.G.LS falls below SW.L.T the control will turn off both

Capacity Clamp mode and Limiting mode with one exception.

If (LAT – LAT last time through the capacity calculation) is

greater than “RISE” degrees F per second, the control will stay

in the Capacity Clamp mode.

If control is in the Limiting mode and then S.G.LS falls

below SW.L.T, and LAT is not rising quickly, the control will

run the capacity calculation routine immediately and allow a

full stage to come back on if desired this first time through

upon recovery. This will effectively override the “max capacity

stage” clamp.

In addition to the above checks, it is also possible at low cfm

for the supply-air temperature to rise and fall radically between

capacity calculations, thereby impacting the limit switch tem-

perature. In the case where supply-air temperature (LAT) rises

above the control point (HT.C.P) + the cutoff point (LAT.L) the

control will run the capacity calculation routine immediately

and drop a stage of heat. Thereafter, every time the capacity

calculation routine runs, provided the control is still in the LAT

cutoff mode condition, a stage will drop each time through.

Falling back below the cutoff point will turn off the LAT cutoff

mode.
INTEGRATED GAS CONTROL BOARD LOGIC — All gas

heat units are equipped with one or more integrated gas control

(IGC) boards. This board provides control for the ignition sys-

tem for the gas heat sections. On size 030-050 low heat units

there will be one IGC board. On size 030-050 high heat units

and 055-105 low heat units there are two IGC boards. On size

055-105 high heat units there are three IGC boards. When a

call for gas heat is initiated, power is sent to W on the IGC

boards. For standard 2-stage heat, all boards are wired in paral-

lel. For staged gas heat, each board is controlled separately.

When energized, an LED on the IGC board will be turned on.

See Table 51 for LED explanations. Each board will ensure

that the rollout switch and limit switch are closed. The induced-

draft motor is then energized. When the speed of the motor is

proven with the Hall Effect sensor on the motor, the ignition

activation period begins. The burners ignite within 5 seconds.

If the burners do not light, there is a 22-second delay before an-

other 5-second attempt is made. If the burners still do not light,

this sequence is repeated for 15 minutes. After 15 minutes have

elapsed and the burners have not ignited then heating is locked

out. The control will reset when the request for W (heat) is tem-

porarily removed. When ignition occurs, the IGC board will

continue to monitor the condition of the rollout switch, limit

switches, Hall Effect sensor, and the flame sensor. Forty-five

seconds after ignition has occurred, the IGC will request that

the indoor fan be turned on. The IGC fan output (IFO) is

connected to the indoor fan input on the MBB which will

indicate to the controls that the indoor fan should be turned on

(if not already on). If for some reason the overtemperature limit

switch trips prior to the start of the indoor fan blower, on the

next attempt the 45-second delay will be shortened by 5 sec-

onds. Gas will not be interrupted to the burners and heating will

continue. Once modified, the fan delay will not change back to

45 seconds unless power is reset to the control. The IGC boards

only control the first stage of gas heat on each gas valve. The

second stages are controlled directly from the MBB board. The

IGC board has a minimum on-time of 1 minute. In modes such

as Service Test where long minimum on times are not

enforced, the 1-minute timer on the IGC will still be followed

and the gas will remain on for a minimum of 1 minute.

Table 47 — Staged Gas Heat

Table 48 — Staged Gas Heat Control Steps (HT.ST = 0)

NUMBER OF

STAGES

HT.ST

CONFIGURATION

UNIT SIZE

48Z

HEAT SIZE

030-050

Low

030-050

High

055-105

Low

055-105

High

STAGE

RELAY OUTPUT

CAPACITY

Heat 1

Heat 2

Heat 3

Heat 4

Heat 5

Heat 6

MBB-RLY8

MBB-RLY7

SCB-RLY1

SCB-RLY2

SCB-RLY3

SCB-RLY4

IGC1

MGV1

IGC2

MGV2

IGC3

MGV3

OFF

100

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